1. Field of the Invention
The present invention relates to electronic transformer systems for powering gaseous discharge tubes and, in particular, to high power factor systems of that kind.
2. Description of Prior Art
Electronic transformers are known to be a competitive alternative to the traditional core-and-coil transformer system for powering gaseous discharge tubes. Since such electronic transformers usually operate at frequencies of tens of kilohertz or higher, they have the advantages of smaller size, lighter weight, and better efficiency compared to core-and-coil transformers.
Today, a majority of the electronic transformers for gaseous discharge tubes use main lines as their power source, that is, the standard AC line voltage. Such transformers usually have a low power factor and cause a considerable amount of input harmonics. These transformers usually use a rectifier stage to convert AC line power to a DC voltage and use an inverter stage to convert the DC power to high frequency AC power to drive a gaseous discharge tube. A capacitor of tens or hundreds of microfarads is usually connected to the output of the rectifier stage, so that a smooth DC voltage can be obtained for the inverter stage. Nevertheless, because of this capacitor, the input current flow is blocked for a large portion of each cycle. The blockage results in an input current waveform having high current pulses, severely distorted from sinusoid. As a result, the power factor of such transformers is usually 0.4 to 0.6, which means only 40 to 60 percent of the available power is utilized.
Some power factor correction stages have been devised in the past to improve this low power factor. The most commonly used power factor correction stage in a DC power supply is essentially a boost or buck/boost converter with rectified AC, having 120 Hz ripple as input to provide a smoother DC output. Such a power factor correction stage could be added to an electronic transformer to obtain higher power factor. Nevertheless, an additional power factor correction stage introduces extra cost and reduces the efficiency of the transformer.
The invention disclosed herein provides a cost-effective apparatus to achieve a high power factor for an electronic transformer for gaseous discharge tubes.